In general, complex man-made structures, whether stationary such as buildings and bridges, or mobile such as moving vehicles operating on land, sea, air or space, are normally made from many components attached together forming a complex structure. The design of attachment points, commonly known as joints, requires special knowledge and skill for engineering design and analysis. A major part of this task is the selection of proper components, such as fasteners, for joining and fastening the structural components together.
The main purpose and primary objective in joint design is to facilitate the load transfer from one component of the structure to another component. The joined structure should be able to sustain the external and internal loads that may be experienced while performing its intended function. Loading may be in sustained static form or in a variable dynamic form. The functioning environment may be corrosive in nature, affecting the material properties and integrity of the fasteners and structural material. The operating environment may also undergo temperature changes affecting the load carrying characteristics of the joint and fasteners. All of these factors should be considered in joint design and fastener selection.
Since man's original venture into building structures and moving vehicles, many types of fasteners have been conceived, developed, and used successfully. However, with an ever developing civilization, the need for continuous improvement is always evident. One common feature in most joint designs is to create holes, or apertures, in the joint components, typically referred to as work pieces, to insert and attach the components to each other by placing a suitable fastener in the matching holes. These fasteners, referred to by many different names and terms, for example Blind Fasteners, are major contributors for constructing buildings, tools, vehicles, and other important structures comprising the present form of civilization and physical life.
With the demand for lightweight, high strength aerospace structural components, the usage of composite materials was necessitated. Composite materials are composed of at least two major components: load carrying fibers and a bonding matrix. The load carrying components are made from high strength fibers, such as carbon fibers, while the bonding matrix is normally made from nonmetallic materials, such as epoxy, having much less mechanical strength. Unlike homogenous metallic structures having ductility, the fibrous nature of composite material exhibits non-homogenous mechanical properties, thus complicating the process of efficient load transfer at mechanical joints. As is well known to those skilled in the art of fastening, efficient load transfer is accomplished when the structural material exhibits a certain degree of compliance and resiliency. Metallic structures normally exhibit resiliency and compliance, but the composite materials, lacking adequate ductility, are brittle in nature and are subject to unpredictable brittle type failure at the structural joint.
The brittle nature and the lack of resiliency of composite materials will often promote non-uniform distribution of loads to multiple fasteners installed in a single joint. The installation loads required for installing ordinary blind fasteners will often generate high levels of compressive stresses around the fastener holes of the structure. These compressive forces, when directly applied on composite structures, cause damage in the form of cracks, delamination, and fiber breakage, which adversely affects the load carrying capability of the structure, specifically around the holes in the structure. These types of damages and flaws need to be minimized.
The issue of proper distribution and sharing of the load between the fasteners and the structural components of the joints having multiple fasteners may be partially achieved by precision drilling for producing close tolerance holes and implementing a process of perfect hole alignment, such as precision match drilling of the holes. However, these solutions are expensive and difficult to achieve in practice. Another approach for addressing these issues is to utilize a hole-filling type fastener design. Composite structures, however, typically do not tolerate hole expansion readily, as excessive hole expansion tends to cause delamination and cracks in the structure. Therefore, while a fastener with hole-filling capability is desirable, a fastener design which creates excessive hole expansion in the structure needs to be avoided.
The Blind fasteners were invented to simplify the installation process and address the issues of restricted accessibility. The term “blind fastener” signifies the feature which allows the fastener to be installed from one side of the structure, thus accommodating for installation applications where only one side of the structure is accessible. As a result of being able to be installed from just one side of the structure, a single operator, with the aid of specialized tools, can install the fastener in the structure quickly and effectively, thus reducing installation costs, such as costs associated with labor. With these types of fasteners, proper formation of the blind side upset head is critical to the fastener performance. In particular, blind fasteners, when installed, are expected to form a well-defined upset head against the back sheet of a work piece within the expected grip range specified by the fastener design where “grip” is the thickness of the work piece, with the “grip range” being defined by the maximum grip and minimum grip specified for the specific fastener. A common value for the grip range is one-sixteenth ( 1/16) of an inch.
Many types of blind fasteners have been invented and are being used in significant numbers for attaching all types of structures, especially for attaching aerospace structural components where space and accessibility is restricted. Blind fasteners may be categorized based on their shear strength as design requirements for shear strength dictate whether the fastener is categorized (or known) as a Blind Rivet or a Blind Bolt. In the case of a Blind Rivet, typical shear strength is 50 ksi, or 50,000 pounds per square inch, which is sufficient enough to replace solid rivet applications, while in the case of a Blind Bolt, the shear strength is typically in excess of 90 ksi, which is sufficient to replace a typical nut and bolt application.
Design features related to the installation and grip accommodation method of the fastener determine whether the Blind Fastener is known as a Wiredraw Fastener (i.e., fastener pin elongates, due to a wire draw action within the sleeve), a Shear Ring Fastener (i.e., fastener pin utilizes a shear ring which breaks at a predetermined load), or a Variable Sleeve Hardness Fastener (i.e., fastener sleeve buckles onto the work piece). Blind fasteners, no matter which category they fall into, typically share many traits. Self-locking blind fasteners are normally comprised of a sleeve, a pin, and a lock collar. The sleeve may be comprised of an enlarged manufactured head of specific design, normally either a protruding head or flush head design. The pin may be designed with an enlarged preformed head and pin tail section, all designed to fit within the sleeve during assembly, and advanced to a predefined position by either a pulling motion or a turning procedure, during the installation process. After completion of the locking process, the tail portion of the pin may be broken off and discarded. A lock collar may be designed to retain the pin within the sleeve and secure the pin and sleeve together. By fitting and deforming the lock collar into cavities created upon the correct positioning of the pin within the sleeve, the installed fastener exhibits the expected strength requirements.
Currently, prior art Blind Fasteners require a preformed enlarged head to interact with the sleeve and form an upset head upon installation. These Blind Fasteners must therefore be assembled by passing the pin tail through the blind end of the sleeve. Thus, the diameter of the pin tail is limited by the internal diameter of the sleeve. As a result, the strength of the pin tail and pull force that can be applied during the installation process is limited by the size required for assembly.
Consequently, a new fastener design, which alleviates the problems inherent in conventional fasteners, is needed.